I made these ages ago, they go quite well in a Blast Bazooka with the the restrictor cut out but without a new barrel. They don't last very long due to being hollow and made of shitty foam.
You could put a clip in the handle, you'd just have to move it forward and use the same mechanism that bolt used in the FAR. Also, has anyone ever considered using a drum magazine like those found on tommy-guns and WWI-era bomber turrets?
I know it's just a belt inside a case but still, that'd be a lot more space-efficient than a vertical clip.
If you want to repair cracks etc in the airtank pump it up and put it under water. Wherever bubbles appear you need to put epoxy or plumbers goop.
Barrel rotation could be a simple build-up of crud from sitting in your basement, open the damn thing up and fiddle around.